Human tumor necrosis factors alpha (TNF-alpha) and beta (TNF-beta or lymphotoxin) are related members of a broad class of polypeptide mediators, which includes the interferons, interleukins and growth factors, collectively called cytokines (Beutler, B. and Cerami, A., Annu. Rev. Immunol., 7:625–655, 1989).
Tumor necrosis factor (TNF-alpha and TNF-beta) was originally discovered as a result of its anti-tumor activity, however, now it is recognized as a pleiotropic cytokine capable of numerous biological activities including apoptosis of some transformed cell lines, mediation of cell activation and proliferation and also as playing important roles in immune regulation and inflammation.
To date, there are nine known members of the TNF-ligand superfamily, TNF-alpha, TNF-beta (lymphatoxin-alpha), LT-beta, OX40L, FASL, CD30L, CD27L, CD40L and 4-1BBL. The ligands of the TNF ligand superfamily are acidic, TNF-like molecules with approximately 20% sequence homology in the extracellular domains (range, 12%–36%) and exist mainly as membrane-bound forms with the biologically active form being a trimeric/multimeric complex. Soluble forms of the TNF ligand superfamily have only been identified so far for TNF, LT-alpha, and FasL (for a general review, see Gruss, H. and Dower, S. K., Blood, 85 (12):3378–3404 (1995)), which is hereby incorporated by reference in its entirety.
These proteins are involved in regulation of cell proliferation, activation, and differentiation, including control of cell survival or death by apoptosis or cytotoxicity (Armitage, R. J., Curr. Opin. Immunol., 6:407 (1994) and Smith, C. A., Cell, 75:959 1994).
TNF is produced by a number of cell types, including monocytes, fibroblasts, T cells, natural killer (NK) cells and predominately by activated machrophages. TNF-alpha has been reported to have a role in the rapid necrosis of tumors, immunostimulation, autoimmune disease, graft rejection, resistance to parasites, producing an anti-viral response, septic shock, growth regulation, vascular endothelium effects and metabolic effects. TNF-alpha also triggers endothelial cells to secrete various factors, including PAF-1, IL-1, GM-CSF and IL-6 to promote cell proliferation. In addition, TNF-alpha up-regulates various cell adhesion molecules such as E-Selectin, ICAM-1 and VCAM-1.
The first step in the induction of the various cellular responses mediated by the members of the TNF ligand superfamily is their binding to specific cell surface receptors. The TNF receptor superfamily contains at present ten known membrane proteins and several viral open reading frames encoding TNFR-related molecules. The p75 low-affinity Nerve Growth Factor (NGF) receptor was the first cloned receptor of this family (Johnson, D. et al. Cell, 47:545 (1986). Subsequently, cloning of two specific receptors for TNF show that they were related to the NGF receptor (Loetscher, H. et al., Cell, 61:351 (1990)). In recent years, a new type I-transmembrane TNF receptor superfamily has been established. This family includes the p75 nerve growth factor receptor, p60 TNFR-I, p80 TNFR-II, TNFR-RP/TNFR-III, CD27, CD30, CD40, 4-1BB, OX40 and FAS/APO-1. In addition, several viral open reading frames encoding soluble TNF receptors have been identified, such as SFV-T2 in Shope fibroma virus (Smith, C. A. et al., Biochem. Biophys. Res. Commun., 176:335, 1991) and Va53 or SaIF19R in vaccinia virus (Howard, S. T., Virology, 180:633, 1991). These receptors are characterized by multiple cysteine-rich domains in the extracellular (amino-terminal) domain, which have been shown to be involved in ligand binding. The average homology in the cysteine-rich extracellular region between the human family members are in the range of 25 to 30%.
Inflammation, which is characterized by redness, swelling, heat, and pain, is an essential immune response which occurs following tissue injury or infection. The initial event triggers an elaborate signaling cascade which results in increased local blood flow, blood clotting, and vascular permeability. These acute changes facilitate the recruitment of phagocytic leukocytes to the site of injury or infection. Once at the affected site, the immune cells can begin to neutralize pathogens and contribute to tissue repair.
Among the many protein classes involved in the inflammatory response are blood clotting factors, vasodilating substances (such as histamine and bradykinin), cell adhesion molecules, cytokines (such as interleukins and chemokines), and immune system effector cells (such as neutrophils, macrophages, and lymphocytes).
Although the inflammatory response is an important defense mechanism against infection by foreign substances, inappropriate or excessive activation of inflammation can lead to tissue damage and even death. Medical conditions resulting from inflammation include, but are not limited to, inflammatory bowel disease, multiple sclerosis, arthritis, asthma, allergies, sarcoidosis, septic shock, gastrointestinal cancers, pancreatitis, dermatitis, gout, systemic lupus erythematosis, and Grave's disease. Inflammation is also a potentially life-threatening complication of cardiopulmonary bypass surgery, renal ischemia-reperfusion, and traumatic injury.
Several steroidal and nonsteroidal drugs have been used to control inflammation or to provide symptomatic relief. However, these therapies can be accompanied by numerous side effects which limit their usefulness. Therefore, there is a continuing need for more effective and less toxic alternatives for modulating the inflammatory response.
Clearly, there is also a need for factors that regulate activation, and differentiation of normal and abnormal cells. There is a need, therefore, for identification and characterization of such factors that modulate activation and differentiation of cells, both normally and in disease states. In particular, there is a need to isolate and characterize additional TNF ligands akin to members of the TNF ligand super-family that control apoptosis of transformed cell lines, mediate cell activation and proliferation and are functionally linked as primary mediators of immune regulation and inflammatory response, and, among other things, can play a role in preventing, ameliorating or correcting dysfunctions or diseases.